When Are Materials Susceptible to Stress Corrosion Cracking: Understanding the Causes and Prevention

Stress corrosion cracking (SCC) is a complex phenomenon that can lead to the catastrophic failure of components and structures in various industries, including chemical processing, oil and gas, power generation, and aerospace. Understanding the factors that contribute to SCC susceptibility is crucial for selecting the right materials, designing robust systems, and implementing effective prevention strategies. In this comprehensive blog post, we will delve into the intricacies of when materials are susceptible to SCC, explore the underlying causes, and discuss proven methods for prevention.

Material Susceptibility to Stress Corrosion Cracking

The type of material used is a primary factor in determining its susceptibility to SCC. Different materials exhibit varying levels of resistance to this form of corrosion, and the selection of an appropriate material is crucial in mitigating the risk of SCC.

Austenitic Stainless Steels

Austenitic stainless steels, such as 304 and 316 grades, are commonly used in chemical plants, refineries, and other industrial applications due to their excellent corrosion resistance. However, these materials are particularly susceptible to SCC in certain temperature ranges and in the presence of specific corrosive chemical species. For example, austenitic stainless steels can experience SCC in the presence of chloride ions, which are often found in seawater environments.

According to a study by the Nickel Institute, the susceptibility of austenitic stainless steels to SCC is influenced by factors such as the chromium and nickel content, the presence of sensitizing heat treatments, and the level of residual stress in the material. The study found that increasing the nickel content and reducing the chromium content can help mitigate the risk of SCC in these steels.

Martensitic Stainless Steels

Martensitic stainless steels, such as 410 and 17-4 PH grades, are also susceptible to SCC, particularly in the presence of hydrogen sulfide (H2S) and other corrosive environments. These materials are commonly used in oil and gas production, where they may be exposed to H2S-containing fluids.

Research conducted by TWI Ltd. has shown that the susceptibility of martensitic stainless steels to intergranular SCC can be influenced by factors such as the heat treatment process, the presence of inclusions, and the level of residual stress in the material. Proper material selection, heat treatment, and stress management are crucial in preventing SCC in these steels.

Aluminum Alloys

Aluminum alloys, particularly those containing copper, magnesium, or zinc, can also be susceptible to SCC. The susceptibility of aluminum alloys to SCC is influenced by factors such as the alloy composition, the heat treatment, and the presence of residual stresses.

A study by NASA found that the susceptibility of aluminum alloys to SCC can be reduced by using alloys with lower copper content, such as 2024-T3 and 7075-T6, and by minimizing residual stresses through proper manufacturing and assembly processes.

Service Environment and SCC Susceptibility

The service environment in which a material operates can also significantly contribute to its susceptibility to SCC. The presence of specific chemical species, temperature, and other environmental factors can interact with the material to promote the initiation and propagation of SCC.

Chloride Ions

Chloride ions, commonly found in seawater and some industrial environments, are a well-known contributor to SCC in materials such as austenitic stainless steels. The presence of chloride ions can lead to the formation of localized corrosion cells, which can initiate and propagate SCC.

According to the Nickel Institute, the susceptibility of austenitic stainless steels to SCC in the presence of chloride ions is influenced by factors such as the chloride concentration, temperature, and the presence of other ions or contaminants in the environment.

Hydrogen Sulfide (H2S)

Hydrogen sulfide (H2S) is a common corrosive species found in oil and gas production environments, and it can contribute to the SCC of materials such as martensitic stainless steels. The presence of H2S can lead to the formation of hydrogen-induced cracking, which can then propagate through the material under the influence of stress.

Research by TWI Ltd. has shown that the susceptibility of martensitic stainless steels to SCC in H2S-containing environments is influenced by factors such as the material composition, heat treatment, and the level of residual stress in the component.

Temperature

Temperature is another critical factor that can influence the susceptibility of materials to SCC. In general, higher temperatures can accelerate the kinetics of SCC, leading to faster crack initiation and propagation.

For example, austenitic stainless steels are particularly susceptible to SCC in the temperature range of 50°C to 150°C (122°F to 302°F), where the material’s passivity can be disrupted, and localized corrosion can occur. Maintaining the operating temperature outside of this range can help mitigate the risk of SCC in these materials.

Stress and Strain Contribution to SCC

The presence of stress or strain on a material can also contribute to its susceptibility to SCC. Both residual stresses and applied stresses can play a role in the initiation and propagation of SCC.

Residual Stresses

Residual stresses can arise from various manufacturing and fabrication processes, such as welding, forming, or heat treatment. These residual stresses can act as the driving force for SCC, even in the absence of externally applied loads.

According to the NASA standards, the level of residual stress in a material can be influenced by factors such as the welding process, the joint design, the post-weld heat treatment, and the surface finishing techniques. Proper control and management of residual stresses are crucial in preventing SCC.

Applied Stresses

In addition to residual stresses, the application of external loads or pressures can also contribute to the susceptibility of a material to SCC. The crack propagation in SCC is primarily driven by static or slowly varying stresses, rather than cyclic or dynamic stresses.

The NASA standards emphasize the importance of designing components and structures to minimize the level of tensile stress, as this can help reduce the risk of SCC. This can be achieved through the use of appropriate design practices, such as stress analysis, load distribution, and the incorporation of stress-relieving features.

Prevention Strategies for Stress Corrosion Cracking

To mitigate the risk of SCC, a comprehensive approach involving material selection, environmental control, and stress management is essential. Here are some proven prevention strategies:

1. Material Selection: Choose materials with inherent resistance to SCC, such as nickel-based alloys or duplex stainless steels, which have higher resistance to chloride-induced SCC compared to austenitic stainless steels.

2. Environmental Control: Limit the presence of corrosive chemical species, such as chloride ions or hydrogen sulfide, in the service environment. This can be achieved through the use of protective coatings, liners, or the implementation of effective water treatment and purification systems.

3. Temperature Control: Maintain the operating temperature outside the susceptible range for the material, as identified in the previous sections. This can help preserve the material’s passivity and reduce the risk of localized corrosion.

4. Stress Management: Minimize the level of tensile stress in the component or structure through proper design, manufacturing, and assembly practices. This can include the use of compressive residual stresses, stress-relieving heat treatments, or the incorporation of stress-reducing features in the design.

5. Monitoring and Inspection: Implement regular monitoring and inspection programs to detect the early signs of SCC, such as surface cracks or pitting corrosion. This can help identify potential issues before they escalate into catastrophic failures.

6. Weld Overlay Cladding: As demonstrated by the research conducted by TWI Ltd., the use of weld overlay cladding can be an effective method for protecting stainless steel components, such as pipelines and pressure vessels, against corrosive fluids and the risk of SCC.

7. Hydrogen Embrittlement Prevention: For materials susceptible to hydrogen-induced cracking, such as martensitic stainless steels, measures to prevent hydrogen embrittlement, such as the use of cathodic protection or hydrogen trapping, can help mitigate the risk of SCC.

By understanding the factors that contribute to SCC susceptibility and implementing a comprehensive prevention strategy, engineers and material scientists can ensure the safe and reliable operation of components and structures in various industries.

References

1. The Welding Institute. (2022). Stress Corrosion Cracking – What Factors Cause and Prevent It? Retrieved from https://theweldinginstitute.com/Stress-Corrosion-Cracking-What-Factors-Cause-and-Prevent-It
2. Nickel Institute. (2010). Preventing stress corrosion cracking of austenitic stainless steels in chemical plants. Retrieved from https://www.nickelinstitute.org/media/1766/preventingstress_corrosioncrackingofausteniticstainlesssteelsinchemicalplants_10066_.pdf
3. NASA Standards. (2000). MSFC-STD-3029.pdf. Retrieved from https://standards.nasa.gov/sites/default/files/standards/MSFC/A/0/MSFC-STD-3029.pdf
4. TWI Ltd. (n.d.). Stress Corrosion Cracking. Retrieved from https://www.twi-global.com/technical-knowledge/job-knowledge/stress-corrosion-cracking-064